Effects transitions in a music and audio playback system

ABSTRACT

A music and audio playback system enables an operator to control an audio effect for transitioning an audio output signal from a first audio source to a second audio source by using a single cross-fader control. The transition audio effect combines the two incoming sources using a function that depends on the current position of the cross-fader control. Effects include cross-fading the frequency range, band-ducking, vocoder effects, and beat-cutting effects. The technique is especially advantageous in DJ performance systems in which audio effect-based transitions can add excitement and variety to a performance. Dedicated transition effect preset buttons located near the cross fader facilitate rapid selection by the DJ of the cross-fader controlled transition audio effect.

BACKGROUND

One type of musical performance involves playing back recorded music andsounds in a new “mix.” The performer is commonly called a disc jockey,or DJ.

A DJ relies on many tools to create a musical performance. For example,vinyl records may be manipulated on multiple turntables, or multiplecompact discs (CDs) may be manipulated to create a variety of soundeffects. The disc jockey may vary the speed, pitch, direction, andsequence of playback, and may apply a variety of effects to the playedback music.

More recently, DJs have been using tools that manipulate audio datastored in data files on computers. Some of these tools have beendeveloped with the idea of computerizing what is usually done withturntables, vinyl records, and CD players. The flexibility of thecomputer enables new tools and techniques to be explored for use inperformances. One such computer-based system is described in U.S. PatentPublication No. 2008/0013757, which is wholly incorporated herein byreference. Commercially available computer-based DJ playback systems,such as the Torq® system available from Avid Technology, Inc., ofBurlington, Mass., incorporate expanded functionality enabled byreplacing vinyl record turntables and CD players with simulatedturntables that control digital audio files.

However, DJs continually seek new tools to help make their performancesricher and more compelling. Additional tools need to be easy to use,with a highly responsive interface, since the DJ's hands are alreadyvery busy controlling the audio sources and volumes.

SUMMARY

In a music and audio playback system, the control of an audio effect isassigned to a cross-fader control, enabling a DJ to transition betweentwo audio sources using audio effect-based transitions using a singlecontrol.

In general, under one aspect, a music and audio playback systemcomprises: a first input for receiving audio data from a first audiosource; a second input for receiving audio data from a second audiosource; a processor for processing audio data; an audio output foroutputting audio data; and a user-operable cross-fader control; whereinthe processor is configured to apply in real time a user-selected audioeffect for combining audio data received from the first audio source viathe first input and audio data received from the second audio source viathe second audio input to generate a processed audio signal, wherein aparameter of the audio effect is controlled by a current state of thecross-fader control, and wherein the music and audio playback systemoutputs the processed audio signal via the audio output.

Various embodiments include one or more of the following features. Themusic and audio playback system includes one or more input controls forenabling the user to select the audio effect. The current state of thecross-fader control corresponds to one of a range of positions, therange of positions having a left limit and a right limit, and theprocessed audio signal comprises the audio data received from the firstaudio source when the cross-fader is in the left limit position and theaudio data received from the second audio source when the cross-fader isin the right limit position. The processed audio signal in a firstfrequency range comprises the audio data lying within the firstfrequency range received from the first audio source, and for a secondfrequency range that comprises frequencies not included in the firstfrequency range, the processed audio signal includes the audio datalying within the second frequency range received from the second audiosource. The first frequency range ranges from a lowest frequency presentin either of the audio sources to a frequency f, wherein f is a functionof the current state of the cross-fader. The processed audio signalcorresponds to a frequency cross-fade between the first audio source andthe second audio source, wherein the current state of the cross-fadercontrols one or more frequency cut-offs between frequencies derived fromthe first audio source and frequencies derived from the second audiosource. The music and audio playback system includes a user-operabledirection switch, and switching the direction switch from a first stateto a second state causes the first audio input and the second audioinput to be interchanged before applying the user-selected audio effect.The music and audio playback system comprises a disc jockey performancesystem, or may be part of disc jockey performance system.

In general, under another aspect, a music and audio playback systemcomprises: a first input for receiving audio data from a first audiosource; a second input for receiving audio data from a second audiosource; a processor for processing audio data; an audio output foroutputting audio data; and a user-operable cross-fader control; whereinthe processor is configured to apply in real time a user-selected audioeffect for combining audio data received from the first audio source viathe first input and audio data received from the second audio source viathe second audio input to generate a processed audio signal, and whereinthe music and audio playback system outputs a signal comprised of acombination the processed audio signal at a first level, the audio datafrom the first audio source at a second level, and the audio data fromthe second audio source at a third level, wherein the first, second, andthird levels are controlled by a state of the cross-fader control.

Various embodiments include one or more of the following features. Thesystem includes a user-operable direction switch, and switching thedirection switch from a first state to a second state causes the firstaudio input and the second audio input to be switched before applyingthe user-selected audio effect. The user-selected audio effect includesmodulating a volume of the first audio source with a function of avolume of the second audio source. The modulating function is adecreasing or an increasing function the volume of the first audiosource. The user-selected audio effect comprises a vocoder effect. Theuser-selected audio effect comprises generating a toggle signalcomprised of toggling back and forth between the first audio source andthe second audio source, wherein a speed of toggling back and forthbetween the first audio source and the second audio source is based on atempo of at least one of the first and second audio sources. Prior toapplying the real time audio effect, the audio data received from thefirst audio source via the first input and the audio data received fromthe second audio source via the second audio input is transformed intofrequency space, and the real-time audio effect is applied in frequencyspace to the transformed audio data to produce a processed signal, andthe processed signal is transformed back to a time-domain representationprior to being output at the first level.

In general, under a further aspect, a method of generating a processedaudio signal includes: receiving at an audio and music playback systemincluding a processor and a user-operable cross-fader control, audiodata from a first audio source; receiving at the audio and musicplayback system audio data from a second audio source; and, using theprocessor, combining the received audio data from the first audio sourceand the audio data from the second audio source by applying in real timea user-selected audio effect, wherein a parameter of the audio effect iscontrolled by a current state of the cross-fader control, and whereinthe music and audio playback system outputs the processed audio signal.

In various embodiments the audio and music playback system includes oris a part of a disc jockey performance system.

In general under a yet further aspect, a method of generating aprocessed audio signal includes: receiving at an audio and musicplayback system including a processor and a user-operable cross-fadercontrol, audio data from a first audio source; receiving at the audioand music playback system audio data from a second audio source; and,using the processor, combining the received audio data from the firstaudio source and the audio data from the second audio source by applyingin real time a user-selected audio effect, and wherein the music andaudio playback system outputs a signal comprised of a combination theprocessed audio signal at a first level, the audio data from the firstaudio source at a second level, and the audio data from the second audiosource at a third level, wherein the first, second, and third levels arecontrolled by a state of the cross-fader control. In various embodimentsthe method is performed by a disc jockey using a disc jockey performancesystem.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of selected components of the userinterface of a music and audio playback system.

FIG. 2 a illustrates a process for creating a first type of effectstransition.

FIG. 2 b illustrates a process for creating a second type of effectstransition.

FIG. 2 c shows how certain variables used in the effects transitions aredefined.

DETAILED DESCRIPTION

DJs operate in a highly exacting environment. The control of the audiosources requires split-second timing and control using a separate handfor each audio source. Furthermore, volumes and other settings need tobe set with rapid finger movements without interrupting the control ofthe audio sources. In order to be of value to a DJ, effects should notrequire the use of more than one hand, or even one finger.

In existing DJ systems, the cross-fader controls a fade from a firstsource that is currently playing to a second source by turning up thevolume of the second source while simultaneously turning down the volumeof the first. In the system described herein, the transition between thetwo audio sources is not performed by fading between the volumes of theoutgoing source and the incoming source, but by the application of audioeffects to the audio sources. This feature offers the DJ the ability tovary the performance with audio effect-based transitions by using just asingle easy-to-use control.

An interface for implementing an effects-based transition control isshown in FIG. 1, DJ playback system 102 includes inputs to receivesignals from audio source A (104) and audio source B (106), which may begenerated from digital audio files, or may be generated by playback ofmedia, such as CD or vinyl. The DJ system also includes cross-fader 108,which in its normal function enables the DJ to perform the conventionalvolume-based transition between audio source A and B. In this mode, whenslider 110 is at the far left position, audio source A is played at fullvolume and audio source B is at zero volume; when the slider is at themid-point, each source is attenuated by an equal amount; and when theslider is at the far right position, source A is at zero volume, andsource B is at full volume. In order to activate effects-basedtransitions, the DJ presses one of the effects preset buttons 112. Eachof these buttons assigns a particular predetermined effect to thecross-fader for controlling transitions between the two audio sources.In various embodiments, the effect to be assigned to the cross-fader isselected from a menu of available effects. It is convenient for the DJto preselect the desired effects, so that they can be selected rapidlywith a single press of a button during a live performance. The playbacksystem also includes an audio output to connect the DJ playback systemto audio system 114, such as a headphone for monitoring by the DJ, or asound amplification system for live performance.

The basic methods of generating effects transitions are illustrated inFIGS. 2 a-c. In transitions of Type I, a first process P1 202 is appliedto the first audio source A and a second process P2 204 is applied tothe second audio source B, as shown in FIG. 2 a. Each of the processesoutputs a signal that is a function of both the incoming source signaland the cross-fader state x′, where x′ is defined in FIG. 2 c. Theoutputs of the first process and of the second process are input to gaincontrols 206 and 208 respectively, each of which is controlled by thecross-fader state x′. In the gain controls illustrated in FIG. 2 a, thesolid lines and the dotted lines indicate alternative gain curves; thechoice of curve may depend on the particular transition effect beingapplied, and may be pre-selected by the designer after evaluating thetwo alternatives. The outputs of each of the processes is then combinedto produce the system's output signal. In transitions of Type II,illustrated in FIG. 2 b, audio sources A and B are combined according toa process P that is independent of the cross-fader state. In addition,each of the audio sources are input directly into separate gain controls212, 214, and the output of process P is input to third gain control216, with each of the gain controls applying a gain as a function of thecross-fader state x′.

In each of the transition effects described herein, the effect isprocessed in one of two ways that depends on the user setting of atwo-state physical or software-based switch 116, named DIR, whichdetermines which role is assigned to each of the audio sources inimplementing the audio transition effects. When DIR is set to 0, audiosource A is input to the Audio in A input jack and audio source B isinput to the Audio in B input jack, and x′=x, where x is the absolutecross-fader position, defined over a normalized range of 0 (left end ofthe range) to 1 (right end), and DIR is the direction switch, having avalue of 0 or 1. When DIR is set to 1, the audio sources are swapped atthe input jacks, and the cross-fader state is inverted: x′=1−x. FIG. 2 cillustrates this logic, with the input signals represented as follows:Audio in A₀ is the left cross-fader input stereo audio signal; Audio inB₀ is the right input stereo audio signal.

We now illustrate the application of effect-based transitions with a setof examples.

EXAMPLE 1 Cross-Fading the Frequency Range

In this transition, when transitioning from source A to source B, thefrequency range between a low frequency and a high frequency rangecut-off for audio source A is decreased from the entire frequency rangeto zero, while for audio source B the frequency range is increased fromzero to the entire frequency range. As the transition is firstintroduced, the listener first hears source B in the bass range from 0Hz to a cut-off frequency and source A from the cut-off to the top endof the frequency range. As the transition continues, the cut-offfrequency is increased, and the listener gradually hears source Bfrequencies extending higher to the mid-range, and up, with source Abeing heard only at mid-high frequencies, until the signal from source Bis played over the entire frequency range, and source A is not heard atall. This cross-fading effect corresponds to Type I (see FIG. 2 a), inwhich P1 (202) is a high pass filter having a lower cut-off frequencythat moves from 0 (though in practice, the lower limit is generally inthe range 0.1-10 Hz) to 20 kHz, and P2 (204) is a low pass filter havingan upper cut-off frequency that moves from 0 Hz to 20 kHz as thecross-fader is moved from one end of its range to the other.

In a variant of low pass/high pass filter cross-fading, the cross-faderis used to control two cut-off frequencies that move in oppositedirections. As the cross-fader is moved from one end of the range, afrequency range in the mid-levels (e.g., centered around 1 kHz)gradually expands at both a high frequency cut-off and a low frequencycutoff. The output signal consists of the first audio source for allfrequencies outside the expanding mid-range, and of the second audiosource for frequencies within the expanding mid-range. Referring to FIG.2 a again, in this variant, P1 is a mid-cut filter with two cut-offfrequencies in which the cut-off frequencies move from 1 kHz to 0 Hz and1 kHz to 20 kHz respectively as the cross-fader is moved from one end ofits range to the other.

EXAMPLE 2 Ducker

With the ducker effect, the volume of one of the sources is used tomodulate the volume of the other source. The modulation reduces thevolume of the first source by an amount that is a function of (e.g.,proportional to) the second source. Sliding the cross-fader causes themodulated combination to become more and more pronounced as the firstsignal is faded out until the cross-fader mid-point is reached. Movingpast the mid-point, the second source is faded in over the modulatedfirst source, which is faded out. Thus, when transitioning from source Ato source B, the A signal is increasingly faded out as the modulation ofA by the volume of B is faded in, and the modulated signal reaches amaximum at the mid-point. Moving past the mid-point, the modulatedsignal is faded out as the unprocessed signal B is faded in. This effectbehaves in a similar fashion to a modulated side-chain compressor. Theducker transition effect corresponds to a Type II transition (FIG. 2 b)in which P (210) corresponds to reducing the level of audio source A byan amount that is a function of the instantaneous volume of audio sourceB. Gain controls 212, 214, and 216 are set to combine the output of Pwith the unprocessed signals as illustrated in the Figure.

A variant of the ducker effect is a band ducker, in which the variousband levels of one audio source modulate the corresponding band levelsof the other audio source. Referring again to FIG. 2 b, P is generatedas follows. Audio inputs A and B are split into approximately 12frequency bands. The RMS level of each of the source B bands ismeasured, and a function of this measured value is used to reduce thecorresponding band of source A. In the described embodiment, thefunction is an inverse function, reducing the level of the correspondingA band by an amount proportional to the measured B band level. Thus ahigh level in a given band of B results in a large reduction of thatband level in A. The resulting bands of A are added together to producethe audio output. As for the ducker, the effect of the cross-fader is tovary the strength of this modulation effect, with the modulated signalat a maximum at when the cross-fader is at the mid-point, and fading outin favor of the unprocessed signals A and B respectively as the firstand second ends of the range are approached.

EXAMPLE 3 Vocoder

The vocoder acts in an opposite fashion to the ducker. In the ducker,the levels of the bands in the first signal are reduced to “createspace” for the incoming second signal, whereas the vocoder algorithmattempts to match the signal band levels of both signals. This effectcorresponds to a Type II transition (FIG. 2 b), in which process P isdefined as follows. Audio sources A and B are split into approximately12 frequency bands. The RMS levels of each band of A and B are measured,and a gain is applied to each band of B to make the result match thelevel of the corresponding band of A. In practice, there is a gain limitthat depends on the frequency of a band, since boosting a band by morethan a certain factor, e.g., a factor of 2, can introduce undesirablenoise. In the last step, all the bands of B are added together toproduce the output of the process P. The cross-fader-controlledtransition effect is then applied by mixing the signals A, B, and C asshown in FIG. 2 b.

In a vocoder transition that produces a gentler effect, band-splittingfilters with less steep edges are used to split the sources. Furthersoftening of the effect may be achieved by limiting the gain range withan upper limit not higher than about one.

EXAMPLE 4 Beat Cutter

The DJ playback system is able to determine the tempo of the musicplaying in each of the audio sources. Note, the tempo of each of theinput audio sources is usually the same following tempo adjustment,either automatically or manually. In the described embodiment, thesystem determines the tempo by performing the following steps: (i) cutthe audio file into short time slices, e.g., 5 ms slices; (ii) pass eachof the time slices through low-pass, mid-pass, and high-pass filters;(iii) measure the levels of the slices in the outputs of the low, mid,and high-pass filters; and (iv) search for transients, i.e., levels thatincrease suddenly in one or more of the bands from one slice to thenext. Once transients are detected, the tempo can be determined asfollows: (v) identify characteristics and weightings of transientsassociated with the various kinds of beats, such as downbeats and normalbeats; (vi) based on the identified beat characteristics, estimate thetempo at various different points in the audio file; (vii) analyze allthe tempo estimates to identify the tempo that corresponds to thelargest number of estimates and/or the tempo for which the clearestsignatures were found; and (viii) using the best tempo estimate fromstep (vii), tag all the slices with their beat position.

Using derived tempo information, the system generates a signal bytoggling back and forth between the two audio sources in rhythmicsynchrony with the beat of the music. This transition effect is of TypeII (FIG. 2 b), in which process P implements the rhythmic toggling ofthe sound between A to B. The toggle effect may dwell for an equallength of time with each signal, e.g., a ⅛ note or 1/16 note duration,or dwell for different periods on each signal, e.g., a 1/16 on the Asignal and 3/16 on the B signal. These durations are adapted fordifferent rhythms, and when triplet notes are used. As for all the TypeII transitions, the output of the toggle process is then cross-faded asshown in the FIG. 2 b by mixing the outputs of signals from gain controlA, B, and C. Thus the effect of the cross-fader is as follows:

-   Fader at far left position: audio source A: full; toggle signal:    off; audio source B: off.-   Fader at middle position: audio source A: off; toggle signal: full;    audio source B: off.-   Fader at far right position: audio source A: off; toggle signal:    off; audio source B: full.

EXAMPLE 5 Envelope

In the envelope effect, the envelope of audio signal B is applied toaudio signal A, similar to a one-band vocoder effect.

EXAMPLE 6 Ring Mod

The ring mod transition effect is of Type II in which process P involvesmultiplying signals A and B together. Levels are then compensated for toensure that the result lies within an audible volume range. For example,when both A and B are at a low level, the product level may well be lowenough to require compensation, so that it remains audible and at anappropriate level. This effect produces a noisy, metallic sound, with aclearly artificial character.

EXAMPLE 7 Switch Beat

In this Type I transition effect, P1 is a normal fade except that thealgorithm causes low frequencies in signal A, e.g., less than about 200Hz, to switch more abruptly to the corresponding low frequencies fromsignal B. P2 has the same, but reverse function. In other words, twodifferent cross-fader curves are used for each frequency range. Theincoming signals are cut into two bands, one of which is processed witha normal gain curve, i.e., conventional cross-fade, and the other whichis processed by the “switchy” curve, transitioning rapidly from off tofull on when the cross-fader is in the middle position.

EXAMPLE 8 Loudest

In this Type II transition effect, P outputs the loudest of signals Aand B at any time. Level balancing of A and B may be necessary to obtainoptimal results with this effect. In a variant, the cross-fader is usedto weight the comparison between A and B before the loudness comparisonis made. In another variant, a lower limit is set on the time betweenswitching between the two signals.

EXAMPLE 9 Pump

In this Type II transition effect, signal B is faded up from silence asthe end of each beat approaches. The beat may correspond to a quarternote.

EXAMPLE 10 Limit

In this Type I transition effect, P1 and P2 are limiters instead ofgains. Sliding the cross-fader has the effect of raising the limiterthreshold on B while lowering it on A.

EXAMPLE 11 Halo

In this transition effect, each incoming signal is split into two parts,the first comprising the “mid” parts that are substantially the same inthe stereo left and right channels, and the second “sides” partscomprising the rest. For many musical styles, the mids are principallycomposed of the bass and drums, and the sides of effects sounds,reverbs, and delays. One method of splitting the left (L) and right (R)channels into the mid and sides parts is to use the formula: MID=(L+R)/2and SIDES=(L−R)/2. In this Type I transition, fade-out is first appliedto the mids of signal A, and B is faded in for sides. Continuing alongthe cross-fader range, the A sides are then faded out and the B mids arefaded in. Referring to the FIGS. 2 a, P1 and P2 are applieddifferentially to the mids and sides. This method of band separation isknown as MS processing, especially in the context of vinyl, in which theM (mid) signals are cut vertically into the vinyl, while the S (sides)are cut in horizontally.

EXAMPLE 12 Slow Mo

Signal A is cut into time slices corresponding to the beats; the slicesare progressively lowered in pitch as signal B is faded in. In this TypeI transition effect, P1 corresponds to a slicing and pitch-shiftingfunction, and P2 is a straightforward gain.

In addition to the processes described above, additional Type II audioeffect transitions may be obtained by transforming the input audiosignals into the frequency domain using an FFT algorithm, and performingthe process P in frequency space, The output of the transformationprocess is then resynthesized into the time domain, and passed to thegain controls, as described above in connection with the Type IItransitions.

One example of a frequency space based transition effect is tonalfading. The incoming audio signals are split into two parts, the firstpart containing components of the signal that a listener can identify ashaving a pitch, and the second part containing the rest, such astransients and noise. In one version, a different fading curve isapplied to each of the parts. For example, the tonal part may be fadedout first, leaving “tonal space” for the incoming source, whose tonalpart is faded in first. The non-tonal components are faded in next. In amore sophisticated version, specific tonal information from the firstsource, such as the root key, chords, and the scale, are applied to thecorresponding tonal part of the second source, in a tonal morph effect.

The processing of the described audio transition effects may beimplemented in hardware, and supplied together with a DJ playback systemproduct. Alternatively, various effects may be added to the playbacksystem, such as via a plug-in architecture. Such additional effects, orindeed all the effects, may be executed as software running on a generalpurpose processor or on a DSP within the DJ playback system.

The various components of the system described herein may be implementedas a computer program using a general-purpose computer system. Such acomputer system typically includes a main unit connected to both anoutput device that displays information to a user and an input devicethat receives input from a user. The main unit generally includes aprocessor connected to a memory system via an interconnection mechanism,and may include one or more digital signal processors (DSPs) to assistwith audio signal processing. The input device and output device alsoare connected to the processor and memory system via the interconnectionmechanism.

One or more output devices may be connected to the computer system.Example output devices include, but are not limited to, liquid crystaldisplays (LCD), plasma displays, cathode ray tubes, video projectionsystems and other video output devices, printers, devices forcommunicating over a low or high bandwidth network, including networkinterface devices, cable modems, and storage devices such as disc ortape. One or more input devices may be connected to the computer system.Example input devices include, but are not limited to, a keyboard,keypad, track ball, mouse, pen and tablet, communication device, anddata input devices. The invention is not limited to the particular inputor output devices used in combination with the computer system or tothose described herein.

The computer system may be a general purpose computer system which isprogrammable using a computer programming language, a scripting languageor even assembly language. The computer system may also be speciallyprogrammed, special purpose hardware. In a general-purpose computersystem, the processor is typically a commercially available processor.The general-purpose computer also typically has an operating system,which controls the execution of other computer programs and providesscheduling, debugging, input/output control, accounting, compilation,storage assignment, data management and memory management, andcommunication control and related services. The computer system may beconnected to a local network and/or to a wide area network, such as theInternet. The connected network may transfer to and from the computersystem program instructions for execution on the computer, media data,metadata, review and approval information for a media composition, mediaannotations, and other data.

A memory system typically includes a computer readable medium. Themedium may be volatile or nonvolatile, writeable or nonwriteable, and/orrewriteable or not rewriteable. A memory system typically stores data inbinary form. Such data may define an application program to be executedby the microprocessor, or information stored on the disc to be processedby the application program. The invention is not limited to a particularmemory system. Time-based media may be stored on and input from magneticor optical discs, which may include an array of local or networkattached discs.

A system such as described herein may be implemented in software orhardware or firmware, or a combination of the three. The variouselements of the system, either individually or in combination may beimplemented as one or more computer program products in which computerprogram instructions are stored on a computer readable medium forexecution by a computer, or transferred to a computer system via aconnected local area or wide are network. Various steps of a process maybe performed by a computer executing such computer program instructions.The computer system may be a multiprocessor computer system or mayinclude multiple computers connected over a computer network. Thecomponents described herein may be separate modules of a computerprogram, or may be separate computer programs, which may be operable onseparate computers. The data produced by these components may be storedin a memory system or transmitted between computer systems.

Having now described an example embodiment, it should be apparent tothose skilled in the art that the foregoing is merely illustrative andnot limiting, having been presented by way of example only. Numerousmodifications and other embodiments are within the scope of one ofordinary skill in the art and are contemplated as falling within thescope of the invention.

1. A music and audio playback system comprising: a first input forreceiving audio data from a first audio source; a second input forreceiving audio data from a second audio source; a processor forprocessing audio data; an audio output for outputting audio data; and auser-operable cross-fader control; wherein the processor is configuredto apply in real time a user-selected audio effect for combining audiodata received from the first audio source via the first input and audiodata received from the second audio source via the second audio input togenerate a processed audio signal, wherein a parameter of the audioeffect is controlled by a current state of the cross-fader control, andwherein the music and audio playback system outputs the processed audiosignal via the audio output.
 2. The music and audio playback system ofclaim 1, wherein the music and audio playback system includes one ormore input controls for enabling the user to select the audio effect. 3.The music and audio playback system of claim 1, wherein the currentstate of the cross-fader control corresponds to one of a range ofpositions, the range of positions having a left limit and a right limit,and wherein: the processed audio signal comprises the audio datareceived from the first audio source when the cross-fader is in the leftlimit position; and the processed audio signal comprises the audio datareceived from the second audio source when the cross-fader is in theright limit position.
 4. The music and audio playback system of claim 1,wherein the processed audio signal in a first frequency range comprisesthe audio data lying within the first frequency range received from thefirst audio source, and for a second frequency range comprising onlyfrequencies not included in the first frequency range, the processedaudio signal comprises the audio data lying within the second frequencyrange received from the second audio source.
 5. The music and audioplayback system of claim 4, wherein the first frequency range rangesfrom a lowest frequency present in either of the audio sources to afrequency f, wherein f is a function of the current state of thecross-fader.
 6. The music and audio playback system of claim 1, whereinthe processed audio signal corresponds to a frequency cross-fade betweenthe first audio source and the second audio source, wherein the currentstate of the cross-fader controls one or more frequency cut-offs betweenfrequencies derived from the first audio source and frequencies derivedfrom the second audio source.
 7. The music and audio playback system ofclaim 1 further comprising a user-operable direction switch, and whereinswitching the direction switch from a first state to a second statecauses the first audio input and the second audio input to beinterchanged before applying the user-selected audio effect.
 8. Themusic and audio playback system of claim 1, wherein the music and audioplayback system comprises a disc jockey performance system.
 9. A musicand audio playback system comprising: a first input for receiving audiodata from a first audio source; a second input for receiving audio datafrom a second audio source; a processor for processing audio data; anaudio output for outputting audio data; and a user-operable cross-fadercontrol; wherein the processor is configured to apply in real time auser-selected audio effect for combining audio data received from thefirst audio source via the first input and audio data received from thesecond audio source via the second audio input to generate a processedaudio signal, and wherein the music and audio playback system outputs asignal comprised of a combination the processed audio signal at a firstlevel, the audio data from the first audio source at a second level, andthe audio data from the second audio source at a third level, whereinthe first, second, and third levels are controlled by a state of thecross-fader control.
 10. The music and audio playback system of claim 9further comprising a user-operable direction switch, and whereinswitching the direction switch from a first state to a second statecauses the first audio input and the second audio input to be switchedbefore applying the user-selected audio effect.
 11. The music and audioplayback system of claim 9, wherein the user-selected audio effectincludes modulating a volume of the first audio source with a functionof a volume of the second audio source.
 12. The music and audio playbacksystem of claim 11, wherein the modulating function is a decreasingfunction of the volume of the first audio source.
 13. The music andaudio playback system of claim 11, wherein the modulating function is anincreasing function of the volume of the first audio source.
 14. Themusic and audio playback system of claim 9, wherein the user-selectedaudio effect comprises a vocoder effect.
 15. The music and audioplayback system of claim 9, wherein the user-selected audio effectcomprises generating a toggle signal comprised of toggling back andforth between the first audio source and the second audio source,wherein a speed of toggling back and forth between the first audiosource and the second audio source is based on a tempo of at least oneof the first audio source and the second audio source.
 16. The music andaudio playback system of claim 9, wherein prior to applying the realtime audio effect, the audio data received from the first audio sourcevia the first input and the audio data received from the second audiosource via the second audio input is transformed into frequency space,and the real-time audio effect is applied in frequency space to thetransformed audio data to produce a processed signal, and wherein theprocessed signal is transformed back to a time-domain representationprior to being output at the first level.
 17. A method of generating aprocessed audio signal, the method comprising: receiving at an audio andmusic playback system including a processor and a user-operablecross-fader control, audio data from a first audio source; receiving atthe audio and music playback system audio data from a second audiosource; and using the processor, combining the received audio data fromthe first audio source and the audio data from the second audio sourceby applying in real time a user-selected audio effect, wherein aparameter of the audio effect is controlled by a current state of thecross-fader control, and wherein the music and audio playback systemoutputs the processed audio signal.
 18. The method of claim 17, whereinthe audio and music playback system comprises a disc jockey performancesystem.
 19. A method of generating a processed audio signal, the methodcomprising: receiving at an audio and music playback system including aprocessor and a user-operable cross-fader control, audio data from afirst audio source; receiving at the audio and music playback systemaudio data from a second audio source; and using the processor,combining the received audio data from the first audio source and theaudio data from the second audio source by applying in real time auser-selected audio effect, and wherein the music and audio playbacksystem outputs a signal comprised of a combination the processed audiosignal at a first level, the audio data from the first audio source at asecond level, and the audio data from the second audio source at a thirdlevel, wherein the first, second, and third levels are controlled by astate of the cross-fader control.
 20. The method of claim 19, whereinthe music and audio playback system is associated with a disc jockeyperformance system.